A nitrile such as acrylonitrile or its derivative such as methacrylonitrile, is an important industrial chemical, especially in the plastics, surface coatings, and adhesive industries. For example, acrylonitrile can be used to produce acrylic fiber, as intermediate in the synthesis of antioxidants, pharmaceuticals, dyes, and surface active agents. It can also be used as a modifier for natural polymers or as a pesticide fumigant agent for stored grain.
The production of acrylonitrile or its derivative by the catalytic ammoxidation of an olefin is well known and widely used. For example, the olefin used for producing acrylonitrile is propylene or propane. In this process, the olefin, ammonia and air are reacted over a catalyst at an elevated temperature, producing a vaporous mixture of acrylonitrile, acetonitrile, and hydrogen cyanide, along with water and other side-reaction products. The hot vapor is then cooled and quenched with sulfuric acid to remove unreacted ammonia. The vapor stream is then sent to a recovery system. It is first absorbed in water to create an aqueous stream containing the products of the reaction: acrylonitrile, acetonitrile, and hydrogen cyanide. The aqueous stream is then treated in a series of distillation columns to recover and purify these products. After the acetonitrile is removed for recovery, the hydrogen cyanide is stripped from the acrylonitrile stream, sent to a purification column, and chilled for storage.
Acetic acid is generally introduced during the separation of hydrogen cyanide from acrylonitrile. However, the use of acetic acid in the process has some disadvantages. Some of the acetic acid remains with the acrylonitrile stream, causing corrosion of metal equipment, and potentially remaining as an impurity in the acrylonitrile. In addition, it reacts with residual ammonia to form ammonium acetate, which is then carried by recycle streams back to the first part of the recovery and purification systems where it tends to decompose or release harmful ammonia to the process.
Stronger mineral acids such as phosphoric and sulfuric acids are also often used in processes for the direct production of hydrogen cyanide from methane or methanol as the carbon source. Since these processes produce few or little recoverable byproducts, the mineral acids do not affect the system adversely. However, mineral acids cannot be used in processes where acrylonitrile is the main product because they can react with acrylonitrile, especially under anhydrous conditions such as are found in acrylonitrile purification processes.
Therefore, there is an increasing need to develop a process for the purification of a nitrile such as, for example, acrylonitrile.